The present invention relates to radio frequency antennas and more particularly, to loop antennas which generate fields that are generally canceling at distances of one wavelength or more from the antenna.
In certain known types of electronic systems it is known to provide one or more loop antennas wherein coupling between an antenna and its proximate surrounding is high, but wherein the design of the antenna is such that coupling between the antenna and its distant surrounding (i.e., about one wavelength or more distant from the antenna) is minimized. Such antennas are generally used for near-field communications or sensing applications where the term "near field" means within one half wavelength from the antenna. Examples of such applications include communications with implanted medical devices, short range wireless local area communications networks for computers and radio frequency identification systems including electronic article surveillance (EAS) systems. Generally, the coupling to these loop antennas is primarily via magnetic induction.
For example, radio frequency EAS systems usually include both a transmit antenna and a receive antenna which collectively establish a surveillance zone, and tags which are attached to articles being protected. The transmit antenna generates a variable frequency electromagnetic field within a range of a first predetermined frequency. The tags each include a resonant circuit having a predetermined resonant frequency generally equal to the first frequency. When one of the tags is present in the surveillance zone, the field generated by the transmit antenna induces a voltage in the resonant circuit in the tag, which causes the resonant circuit to generate an electromagnetic field, causing a disturbance in the field within the surveillance zone. The receive antenna detects the electromagnetic field disturbance and generates a signal indicating the presence of the tag (and thus, the protected article attached to the tag) in the surveillance zone.
The design of these antennas should satisfy two objectives: (1) to maximize the coupling to the tag over as wide a distance between the transmit and receive antennas as possible, and (2) to minimize the coupling to the far-field. These are conflicting objectives. Prior art antennas, such as those described by Lichtblau in U.S. Pat. Nos. 4,243,980, 4,260,990 and 4,866,455, herein incorporated by reference, generally incorporate two or more co-planar loops such that in combination the sizes of each loop, the magnitude of the currents within the loops and the direction of the currents generate fields which, when measured at a point distant from the antenna, generally cancel. In other words, the fields created from each of the loops, when summed, net a field which approaches zero. Such far-field cancellation is not possible when only one loop is used. In figure-eight loop antennas, the loops are generally rectangular, arranged in a coplanar configuration, and offset in position such that at least one side of each loop is proximate to a side of another loop. In other words, the shared sides are immediately adjacent to each other.
The present invention provides an antenna having both much reduced far-field coupling properties and increased coupling in a wide area between a transmit antenna and a receive antenna. Generally, the antenna (either transmit or receive) comprises first and second loops of generally equal dimensions and shape wherein the loops are disposed in a fixed position on opposite sides of a central axis extending between the loops. Further, the loops are positioned in separate, spaced, parallel planes. The loops are connected to each other and to either a transmit circuit or a receive circuit by a crossover conductor. Preferably, a length of the crossover conductor from the first loop to the (transmit or receive) circuit is equal to a length of the crossover conductor from the second loop to the circuit. The current in the loops flows in opposite directions and thereby generates substantially canceling fields. The present invention provides an antenna which is highly sensitive to externally emitted signals within a zone proximate to the antenna and highly insensitive to distant emitted signals.